The value of wetlands
In the past 150 years, more than 90% of New Zealand’s wetlands have been drained and destroyed as a result of agriculture and urbanisation. This rate of destruction is among the highest in the world.
The Christchurch-West Melton area has just 73 hectares of its original 4,257 hectares of wetlands left – that’s a loss of more than 98%!
But wetlands are important and widely undervalued ecosystems. They play an essential role in easing the effects human settlement has on our environment. We need to put back some of the wetlands we have lost, before it’s too late.
What are wetlands?
Wetlands are permanently or frequently wet areas, shallow water, and land-water borders that support plants and animals that prefer wet conditions.
They include marshes, estuaries, ponds, swamps, deltas and lagoons. We tend to think of wetlands as places where only grasses, reeds, flaxes, and cabbage trees grow, but wetlands can also be forests. Indeed, kahikatea – our tallest and fastest-growing tree – is right at home in wetlands. Kahikatea grow to heights of 60m or more on the West Coast, and have trunks measuring 2m across. They would naturally grow in some of the wetlands along the Avon River Red Zone, and here they would grow as tall as 30m.
Wetlands provide essential services to both water and land species, as well as to the people living nearby. They protect nearby areas from flood, help clean the water, protect land from erosion, store carbon, provide habitat, and increase biodiversity. They’re also nice places to visit!
Wetlands clean water
Wetlands cleanse water, by trapping sediments and storing nutrients and pollutants such as heavy metals. They then slowly release cleaner water back into the environment – in this case, helping to clean the Avon-Otakaro River.
Red zone wetlands protect us from floods
Wetlands provide important flood protection, by storing flood waters and decreasing peak storm flows, releasing water slowly over time. A network of many small wetlands along a river can store a large amount of water.
Restoring wetlands is an important part of successful and sustainable flood management. It can also be more economically viable than hard engineering solutions such as stop banks (which need continual, and costly, maintenance).
As an example, Waikato’s Whangamarino Wetland has significantly reduced costs and damage to farmland from floods in the lower Waikato River area. During one flood in 1998, it was estimated that the wetland saved $5.2 million in flood control costs.
Wetlands protect Christchurch from coastal erosion
Coastal wetlands protect land from both erosion and coastal storm events. Vegetation helps to bind the shoreline while providing a buffer zone where water can be stored, protecting inland areas from flooding.
Avon-Otakaro forest and wetland park stores carbon
Wetlands are increasingly recognised for taking carbon out of the atmosphere and storing it in plants and soil (known as carbon sequestration).
In 2008 alone, Christchurch produced around 3.6 million tonnes of greenhouse gases. Christchurch City Council aims to reduce net greenhouse gas emissions by 50% by 2050. And in its Climate Smart Strategy, it says carbon sequestration is a key way to mitigate, or balance, carbon emissions.
We all know that forests can store carbon, and indeed the council says it wants to identify land suitable for planting and natural regeneration. Creating Avon-Otakaro Forest Park is one way to do that.
But wetlands could be even better! Recent research by Deakin University suggests wetlands could be up to 50 times more effective than forests in storing carbon – another chance to earn carbon credits and strengthen our city’s ecological credentials.
Ironically, many wetlands were drained to convert land for agriculture. Yet wetlands provide significant economic benefits, far outweighing agricultural land. Wetlands provide the highest annual per-hectare value in New Zealand ($43,320), mainly as a result of their indirect value (for example, water regulation, storm protection, flood control, and drought recovery). In comparison, dairy provides a total annual per-hectare value of $1,796 and sheep and beef $719.
But it’s not just wetlands that can provide huge value and benefit to Christchurch. Much of the land in the Avon River Red Zone would, without human intervention, now be naturally forested. Returning a large tract of native forest to Christchurch would have immense benefits, in terms of cleaning the air, improving health, and – with wetlands – improving our biodiversity.
On the next page: Cleaning our air